In a planetary gear train, a sun gear is surrounded by a plurality of planet gears. The planet gears matingly engage an outer ring gear that encircles the planets.
The outer ring gear is an expensive part to manufacture. Moreover, it is commonly formed of a suitable metal, which contributes unnecessarily to its relatively high weight.
In some applications, such as on an aircraft, weight is at a premium, and steps must be taken to avoid the introduction of undue weight wherever necessary.